The present invention relates to devices used to control cabin pressure in jet airplanes. More particularly, it relates to control logic which generates an outflow valve signal position command, wherein the resultant signal is then used to control an outflow valve that regulates the amount of air exhausted from the cabin.
Regulation of cabin pressure is critical because the human ear is very sensitive to even small pressure changes. Therefore, to prevent discomfort, the cabin pressure control must maintain control in such a way as to prevent rapid changes in pressure. To prevent rapid changes in pressure, the cabin pressure control must be capable of responding quickly to disturbances, such as the skin pressure effect during takeoff rotation or changes in cabin in-flow.
Cabin pressure is also difficult to regulate due to the nature of the cabin response. The cabin responds differently to changes in the outflow valve position depending upon the pressure difference between the cabin pressure and the ambient pressure (outside the plane). The cabin also responds differently depending on cabin volume. For example, the system responds differently if it is used in freight or passenger air crafts.
One early approach for a cabin pressure servo loop used on the 747 and L1011 analog systems was an integral plus lead-lag compensation. The initial digital system simply reproduced the lead-lag logic in digital form. It was found, however, that since the digital system has a finite resolution and a finite time response, because it is not continuous, the net result was an inaccurate reproduction of the lead-lag logic; thus, the desired control response could not be achieved.
These shortcomings led to another form of control logic, known as proportional plus integral logic. This logic also has drawbacks. For example, over-travel, or over-saturation, in the integral path must be controlled in order to avoid problems when the system drives the valve to its position limit. Ideally, the system should come off its limit immediately when the position command reverses in direction. With the proportional and integral components, in order to avoid this problem, extra logic is necessary.
Accordingly, it is the primary object of the present invention to provide a cabin pressure control system that responds quickly to transients and avoids over-saturation, while at the same time maintaining stability within the system.
It is another object to design a system that responds in the same manner, regardless of the cabin-to-ambient differential pressure.
It is still another object to provide a proportional plus integral logic circuit, wherein the integral path can be controlled so as to prevent over-saturation.
It is yet another object to provide a proportional plus integral logic circuit in which integral and proportional gain paths can be separately modified by non-linearities and gains.
It is still a further object of the present invention to design a control system that is easily implemented into software.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.